FIELD OF THE INVENTION
The present invention relates to a high-voltage SOI thin-film transistor, including a semiconductor thin film of a first conductivity type which is embedded in an insulator layer disposed on a semiconductor body, a drain zone and a source zone that are both formed in the semiconductor thin film and have a second conductivity type opposite the first conductivity type, and a gate electrode provided in or on the insulator layer.
High-voltage components, which are suitable for high-frequency power grid or mains applications on the order of magnitude of several hundred kilohertz, for instance in lamp ballasts, preferably employ a dielectric insulation technique with thin insulator layers. or instance, an MIS field effect transistor for high source-to-drain voltages over 100 Volts has already been described in German Patent DE 27 06 623 C2, corresponding to U.S. Pat. No. 4,247,860. In that device, a spacing between a gate electrode and a channel zone increases continuously or in stages toward a drain zone. To that end, in that known MIS field effect transistor, the insulator layer provided under the gate electrode is formed of silicon dioxide in such a way that it becomes thicker toward the drain electrode with increasing distance from the source electrode.
An insulator layer field effect transistor with a drift segment between a gate electrode and a drain zone is also known from German Patent DE 28 52 621 C3. In that device, the drift segment includes an increasing number of dopant atoms beginning at the gate electrode and extending toward the drain zone, so that its dopant concentration rises from the gate electrode to the drain zone. That field effect transistor can thus be used at relatively high operating voltages, without requiring auxiliary voltages and auxiliary voltage sources.
Finally, a semiconductor configuration for high-voltage use is also known from European Patent EP 0 497 427 B1, in which the drift segment is formed as a thin silicon layer embedded in a silicon dioxide insulator layer. The silicon layer has a linearly increasing dopant concentration from the gate electrode toward the drain zone.
The dopant concentration of a drift segment formed as a semiconductor thin film can be substantially higher than the dopant concentration of approximately 10.sup.12 cm.sup.-2 that corresponds to a so-called "breakdown charge". Suitable values for the dopant concentration of the drift segment are accordingly in a range from 5.times.10.sup.12 to 2.times.10.sup.13 cm.sup.-2.